Discontinuous coverage networks are also denoted as discontinuous coverage access networks. According to the working principle of discontinuous coverage networks, transmission of data from a specific network equipment via a radio access point to users as, e.g., mobile stations or terminals, generally occurs only in high rate data transfer areas which benefit from good conditions for radio-based data transfer. The specific network equipment is also referred to in the following as network component and such high rate data transfer areas are hereinafter referred to as ‘transfer zones’. A given transfer zone is generally made up of a number of highest radio data areas (corresponding to the highest physical transmission modes) surrounding a given radio access point.
In the area between two transfer zones is usually a region where only medium and low data transfer rates are available from the network to the mobile terminals. The corresponding zones in which no transmission between access points and users is possible are called ‘no-transmission zones’. Discontinuous coverage networks might provide some additional data transfer in so-called ‘intermediate zones’ located between individual transfer zones by way of especially established transmissions if required to guarantee a particular quality of service (QoS), i.e. avoid service breaks and provide fast service start. An intermediate zone is usually split into two areas, a medium data rate area and a low data rate area. Hereinafter, said no-transmission zones and said intermediate zones will be referred to as ‘no-transfer zones’ in order to distinguish them from the above defined high data rate transfer zones.
A discontinuous coverage network is generally made up of a number of radio access points (RAPs), a respective transfer zone around each radio access point, and a no-transfer zone separating a given radio access point, i.e. the corresponding transfer zone from other radio access points in the network. Thus, the individual transfer zones of the discontinuous network can be referred to as ‘disconnected transfer zones’.
In order to avoid a service break in the no-transfer zone between two transfer zones of the network, in the two kinds of networks described above data is transferred to the mobile station and stored in a respective cache memory of the mobile stations during the crossing of transfer zones. The cached data is then used in the no-transmission or intermediate zones to make an application running on the mobile stations operate seamlessly. Thus, service breaks in the no-transfer zones between transfer zones are mainly avoided owing to cache memories and corresponding caching mechanisms located, e.g., in the above mentioned network component. The network component can be an access controller or a gateway on the network side.
The patent application EP 1 549 096 describes a telecommunication method for a telecommunication network which provides discontinuous coverage. The telecommunication network has a plurality of disconnected cells. A method is disclosed which comprises providing a data stream to a mobile station within the coverage of a first one of the plurality of cells. The data stream is interrupted when the mobile terminal is moved outside the coverage. Further a second one of the plurality of cells to be the target cell of the mobile terminal is predicted and the data stream is provided to a server assigned to the second cell. The data stream is resumed to the mobile terminal when the mobile terminal is within the coverage of the second cell.
The patent application EP 1 624 712 describes a method for improving mobility in discontinuous coverage networks, an access controller and a corresponding radio access point. The method disclosed in EP 1 624 712 provides services from a content server to a terminal belonging to a discontinuous coverage network, wherein the terminal moves from a first radio coverage zone to a second radio coverage zone over a zone of non radio coverage. The discontinuous network architecture comprises access controllers and radio access points, wherein each pair of radio access controllers and radio access points defines a radio coverage zone. The method comprises the step of sending in advance from a first access controller data in a cache memory of at least one radio access point of the second radio coverage zone, when the terminal is outside of the second radio coverage zone.
In order to provide a very low probability of service breaks with this discontinuously service delivery method, i.e. guarantee a given QoS, the caches of mobile terminals leaving a given transfer zone should be filled with data to maximum extent as data is only transferred to the mobile station within the transfer zones. Therefore, in known discontinuous coverage networks robust methods of data packet scheduling (also referred to as lines of cache (LoC)) and suitable medium access control (MAC) protocols typically allow all mobile stations to receive the required lines of cache for a given application before leaving a transfer zone.
A crucial issue for networks serving mobile users is however the handover mechanism. In a traditional way, the connection to the radio access point of the initial transfer zone was broken and a new connection to the radio access point of the new transfer zone was established. This is known as a ‘break before make’ handover or ‘hard’ handover. This technique is employed in first and second generation communication networks. A break before make handover leads to a temporary disconnection of the mobile station to the network which leads to a decrease in the network capacity. The hard handover techniques are optimized in order to minimize the connection interruptions duration and in order to avoid any data loss and in order to reduce the capacity loss of the network.
In third and fourth generation mobile communication networks an alternative solution is nowadays preferred. There, so-called ‘make before break’ or ‘soft’ handovers are carried out in order to transfer a connection that has been initially established between a mobile station and a network via a radio access point to another radio access point. A connection to the new radio access point is created before leaving the current radio access point and then, the old connection is broken. However, the establishment of two parallel transmissions during the soft handover requires higher radio resource usage in order to ensure the same service on both connection lines. Thus soft handovers must be achieved fast to avoid the network capacity diminution.
Soft handover techniques have been developed for networks which have cells that operate on the same frequency noted inter-frequency networks although hard handover techniques have been designed for networks with cells operating on different frequencies noted for inter-frequency networks. Compared with hard handovers, soft handovers require less power which reduces interference. Furthermore, soft handovers provide a continuous service as the connection is never interrupted. Thus soft handovers yield better performances in terms of capacity than hard handover techniques.
But even if the hard handover or soft handover duration is well optimized, the handover mechanism can be damaging to pico cell networks with small cell sizes and/or with short distances between neighboring transfer zones, because mobile stations might be handed over from one cell to a neighboring cell frequently. Thus, the handover mechanism is more often triggered than in networks in which the cell sizes are larger and/or in which the transfer zones lie further apart. A large amount of handovers that have to be handled by the network can lead to a dramatic decrease in the network's capacity because of the numerous triggering of two parallel transmissions or connections interactions for each user changing from a high data rate transfer zone of one cell to another high data rate transfer zone of another cell.
There is therefore a need for an improved method of triggering a handover of a mobile station in a network providing discontinuous coverage. There is furthermore a need for an improved discontinuous coverage network that provides means for performing the method in accordance with the invention and the need for an improved network component for triggering the handover of the mobile station.